1. Field of the Invention
This invention relates to an improved process for removing acidic components from gaseous mixtures containing them and more particularly relates to a method for preventing phase separation of the basic aqueous scrubbing solution at high temperatures and low fractional conversions during the contacting steps.
2. Description of the Prior Art
It is well known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO.sub.2, H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS and oxygen and sulfur derivatives of C.sub.1 to C.sub.4 hydrocarbons with amine solutions to remove these acidic gases. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The acid scrubbing processes known in the art can be generally broken into three (3) categories.
The first category is generally referred to as the aqueous amine process where relatively large amounts of amine solutions are employed during the absorption. This type of process is often utilized in the manufacture of ammonia where nearly complete removal of the acid gas, such as CO.sub.2, is required. It is also used in those instances where an acid gas, such as CO.sub.2, occurs with other acid gases or where the partial pressure of the CO.sub.2 and other gases are low.
A second category is generally referred to as the aqueous base scrubbing process or "hot potash" process. In this type of process a small level of an amine is included as an activator for the aqueous base used in the scrubbing solution. This type of process is generally used where bulk removal of an acid gas, such as CO.sub.2, is desired. This process also applies to situations where the CO.sub.2 and feed gas pressures are high. In such processes, useful results are achieved using aqueous potassium carbonate solutions and an amine activator.
A third category is generally referred to as the nonaqueous solvent process. In this process, water is a minor constituent of the scrubbing solution and the amine is dissolved in the liquid phase containing the solvent. In this process, up to 50% of the amine is dissolved in the liquid phase. This type of process is utilized for specialized applications where the partial pressure of CO.sub.2 is extremely high and/or where many acid gases are present, e.g., COS, CH.sub.3 SH and CS.sub.2.
The present invention pertains to an improved process for practicing the second category of acid scrubbing process described above, namely, the aqueous base scrubbing process or "hot potash" process. Many industrial processes for removal of acid gases, such as CO.sub.2, use regenerable aqueous alkaline scrubbing solutions, such as an amine and potassium carbonate, which are continuously circulated between an absorption zone where acid gases are absorbed and a regeneration zone where they are desorbed usually by steam-stripping. The capital cost of these acid scrubbing processes is generally controlled by the size of the absorption and regeneration towers, the size of the reboilers for generating stripping steam, and the size of the condensers which condense spent stripping steam so that condensate may be returned to the system to maintain proper water balance. The cost of operating such scrubbing plants is generally related to the amount of heat required for the removal of a given amount of acid gas, e.g., thermal efficiency, sometimes expressed as cubic feet of acid gas removed per pound of steam consumed. Means for reducing the costs in operating these industrial processes have focused on the use of absorbing systems or combinations of chemical absorbents which will operate more effeciently and effectively in acid gas scrubbing processes using existing equipment.
There are a number of patents which describe improvements to improve the efficiency of the "hot potash" process. Some of these improvements are described below.
In U.S. Pat. No. 2,718,454, there is described a process for using potash and similar alkali metal salts in conjunction with amines, such as monoethanolamine, diethanolamine and triethanolamine to remove acid gases from a gas mixture. The combination of the alkali metal compounds in conjunction with the designated amine yields higher capacity for acid gases than system with the amines alone.
In U.S. Pat. No. 3,144,301, there is disclosed that use of potassium carbonate in conjunction with diethanolamine and monoethanolamine to remove CO.sub.2 from gaseous mixtures.
In U.S. Pat. Nos. 3,637,345, 3,793,434, and 3,848,057, processes for the removal of acid gases by means of aqueous carbonate scrubbing solutions activated by an amine, such as 1,6-hexanediamine, piperidine and their derivatives are described.
In U.S. Pat. No. 3,856,921, there is disclosed a process for removal of acid gases from fluids by use of a basic salt of an alkali or alkaline earth metal and an amine activator, such as 2-methylaminoethanol, 2-ethylaminoethanol, morpholine, pyrrolidine and derivatives thereof.
U.S. Pat. Nos. 3,563,695, 3,563,696, and 3,642,430 to Benson disclose processes for removing CO.sub.2 and H.sub.2 S from gaseous mixtures by alkaline scrubbing processes wherein at least two separate regeneration zones are provided. Alkanolamines and aminoacids are described as activators, but the use of sterically hindered amines is not taught or disclosed in these patents.
Belgian Pat. No. 767,105 discloses a process for removing acid gases from gaseous streams by contacting the gaseous streams with a solution comprising potassium carbonate and an aminoacid, such as substituted glycines (e.g., N-isopropyl glycine, N-t-butylglycine, N-cyclohexylglycine, etc.). The data in Table V of the patent indicates that the highly substituted compounds, such as N-t-butylglycine, are inferior to the straight chain compounds, such as N-butylglycine.
British Pat. No. 1,063,517 to Allen G. Eickmeyer discloses removal of acid gases, such as CO.sub.2 and H.sub.2 S, from gaseous mixtures by the use of potassium carbonate and other alkali metal carbonates in conjunction with particular amines which will avoid corrosion problems and at the same time accelerate the absorption and subsequent desorption of the CO.sub.2 and H.sub.2 S. Specifically disclosed amines are ethylene polyamine, alkanolamines, or alkanolamine borates as well as mixtures thereof. Examples of such amines are ethylenediamine, diethylenetriamine and diethanolamine.
British Pat. No. 1,218,083 describes a process for removing acid gases from feed streams by contacting the feed streams with a composition comprising an alkaline salt, such as potassium carbonate, and an alkanolamine such as diisopropanolamine.
British Pat. No. 1,238,696 discloses a process for removing acid gases from feed streams by contacting the feed streams with a composition comprising an organic solvent and an alkanolamine, such as cyclohexylaminoethanol. The patent does not disclose the advantages of using sterically hindered amines to improve working capacity.
British Pat. No. 1,305,718 describes a process for removing acid gases from gaseous streams by contacting the same with an absorbing solution comprising a solution of an alkaline salt of an alkali metal and regeneration of the absorbing solution wherein the absorbing solution includes a minor amount of a substituted or unsubstituted aminoacid. Examples of the aminoacids described are N-ethyl-3-amino propionic acid, N-ethyl-4-amino butyric acid, and N-ethyl-6-amino hexanoic acid.
U.S. Pat. No. 2,176,441 to Ulrich et al. teaches the use of aminoacids having a primary, secondary, or tertiary amino group and at least two nitrogen atoms to remove acidic gases. The patentees provide various general formulae for the aminoacids taught to be useful in the acid gas scrubbing process. While certain "sterically hindered amines" can be derived by proper choice of substituent groups in the general formulae there is no teaching that the sterically hindered amines will achieve any unexpected results, such as improved working capacity.
Canadian Pat. No. 619,193 teaches the use of various aqueous solutions containing specific amino compounds for the removal of acidic gases, such as CO.sub.2, from gaseous feed streams. Careful choice of the various R.sub.1 and R.sub.2 groups in the formula in column 4, lines 35-40, will reveal sterically hindered amines. However, there is no teaching that these amines give improved results or working capacity compared to nonsterically hindered amines.
Prior art workers have taught that sterically hindered amines would have low rates of combination with CO.sub.2 and apparently concluded, although other explanations are possible, that such sterically hindered amines would be inefficient in CO.sub.2 scrubbing processes. For example, Sharma, M.M., Trans. Faraday Soc., 61, 681-8 (1965) described the kinetics of reaction between CO.sub.2 and COS with 38 amines, some of which are sterically hindered amines. Other researchers have attributed relatively poor absorption rates of CO.sub.2 by amines to steric hindrance. See, for example, J. L. Frahn and J. A. Mills, Aust. J. Chem., 17, 256-73, 263 (1964) and M. B. Jensen, Acta Chemica Scandinavica, 11, 499-505 (1957).
Shrier and Danckwerts, Ind. Eng. Chem. Fundamentals, 8, 415 (1969) discussed the use of amines as promoters for aqueous carbon dioxide absorption solutions. However, these researchers only ran initial absorption experiments and did not recognize the unique capacity advantages obtained by using sterically hindered amines in an acid gas scrubbing process. Also of interest is Danckwerts and Sharma, The Chemical Engineer, Oct. 1966, pp 244-280.
In the prior art discussed above, it is apparent that the efficiency of processes employed absorbing solutions is generally limited by the relatively slow rate of transfer of molecules of the acid gas from the gas phase to the liquid phase as well as in the regeneration of the absorbing solution. Many of the above-described prior art processes deal with means to render the acid gas scrubbing process more efficient. In copending U.S. application Ser. No. 590,427, filed June 26, 1975, the disclosure of which is incorporated herein by reference, there is disclosed and claimed processes for scrubbing acid gases, e.g., CO.sub.2, comprising the use of sterically hindered amines and the scrubbing solutions, per se. These sterically hindered amines, unexpectedly improve the efficiency, effectiveness and working capacity of the acid gas scrubbing processes in all three of the above-mentioned process categories.
It has been observed, however, that when a sterically hindered amine selected from the group consisting of aminoethers, aminoalcohols, di- and triamines are used as activators,(i.e., the most effective activators in terms of working capacity) in a "hot potash" CO.sub.2 containing acid gas scrubbing process, phase separation often occurs under acid gas-lean conditions, particularly during desorption. This provlem occurs more frequently when the potassium carbonate is used in 30% concentrations. This observed problem of phase separation could be resolved by carrying out an incomplete desorption of the acid gas, so as to maintain the scrubbing solution in the one-phase region. However, such a proposal would not take full advantage of the absorbent (scrubbing) solution's thermodynamic capacity.
It has now been discovered that the above-described phase separation problem can be solved, and full advantage can be taken of the absorbent solution's thermodynamic capacity by adding at least one aminoacid containing 4 to 8 carbon atoms as a cosolvent for the sterically hindered amine activator. This discovery was unexpected since we have found that aminoacids having less than 4 carbons, such as glycine, do not have this cosolvency effect.